1. Field of the Invention
The present invention relates to an input screen scintillator for a radiological image intensifier tube. It also relates to a method of manufacturing such a scintillator.
2. Description of the prior art
Radiological image intensifier tubes are well known in the prior art. They allow a radiological image to be transformed into a visible image, generally for allowing medical observation.
These tubes are formed by an input screen, an electronic optical system and an observation screen.
The input screen comprises a scintillator which converts the incident X ray photons into visible photons. These visible photons then strike a photocathode, generally formed by an alkaline antimonide which, thus excited, generates an electron flow. The photocathode is not deposited directly on the scintillator but on an electrically conducting underlying layer which allows the charges of the material of the photocathode to be reconstituted. This underlying layer may for example be formed of alumina, indium oxide or a mixture of these two substances.
The electron flow from the photocathode is then transmitted by the electronic optical system which focuses the electrons and directs them onto an observation screen formed of a luminograph which then emits a visible light. This light may then be processed, for example, by a television, cinema or photograph system.
The scintillator of the input screen is generally formed of cesium iodide deposited by vacuum evaporation on a substrate. The evaporation may take place on a cold or hot substrate. The substrate is generally formed by an aluminum skull-cap shaped piece with spherical or hyperbolic profile. A thickness of cesium iodide is deposited which is generally between 150 and 500 micrometers.
The cesium iodide is naturally deposited in the form of needles having a diameter of 5 to 10 micrometers. Since its refraction index is 1.8, it benefits from a certain optical fiber effect which minimizes the lateral diffusion of the light generated within the material.
In FIG. 1 an aluminum substrate 1 has been shown schematically having a few cesium iodide needles 2. The aluminum substrate receives a flow of X ray photons symbolized by vertical arrows. There have been shown with broken lines in the Figure examples of paths followed in the cesium iodide needles by the visible radiation corresponding to the incident X ray photons. The normal paths, which bear the reference 3, cause the production of a light signal at the end of the cesium iodide needles. There is also diffusion laterally of the light conveyed by the cesium iodide needles, as is shown in the Figure with the reference 4.
The resolution of the tube depends on the capability of the cesium iodide needles to correctly channel the light. It depends on the thickness of the cesium iodide layer. An increase in thickness causes a deterioration of the resolution. But, on the other hand, the greater the thickness of cesium iodide the more the X rays are observed. A compromise must then be found between the absorption of the X rays and the resolution.
Another factor which influences the resolution of the tube is the heat treatment which the input screen must undergo during manufacture thereof. This treatment takes place immediately after the vacuum evaporation of the cesium iodide. It ensures the luminescence of the screen because of the doping of the cesium iodide by sodium or thalium ions for example. This heat treatment generally consists in heating the screen to a temperature of about 340.degree. C. for about an hour, while placing it in a dry air or nitrogen atmosphere.
The problem which arises is that, during this absolutely obligatory heat treatment, the needles of the scintillator undergo a certain coalescence and agglomerate together, as has been shown schematically in FIG. 2. This coalescence causes greater lateral diffusion of the light, see the broken line arrows bearing the reference 4, and the resolution is deteriorated.
To overcome the coalescence which occurs during the heat treatment, it was proposed in the prior art to form the scintillator of the input screen by alternately evaporating pure cesium iodide and cesium iodide doped with a refractory material. It was hoped that needles thus formed by alternate layers of pure cesium iodide and cesium iodide doped with a refractory material would not come into contact during heat treatment. This solution has not given the expected results.